a) Field of the Invention
This invention relates to a process for producing plastic microballoons (hereinafter simply called xe2x80x9cmicroballoonsxe2x80x9d) which are prevented from flying and are used as a filler in various paints, construction materials, plastics and the like to reduce their weights. Described more specifically, expanded microballoons are generally used after applying non-flying treatment because they are light, have pronounced flying tendency and are hence difficult to handle. Nonetheless, they are still accompanied by various problems, and their handling properties and the like are by no means satisfactory. The present invention is concerned with a process for producing non-flying microballoons with good handling properties.
b) Description of the Related Art
Microballoons are generally packed in containers such as plastic bags or container sacks. When microballoons are taken out of such a container for addition to a base material such as a paint, they fly up in air as they are very light in weight. Upon mixing them with stirring, they rise to the surface of the base material so that a considerable time is required until they are evenly added and mixed in the base material. Microballoons are, therefore, a material the handling of which is extremely difficult.
To overcome these problems, proposals have been made including, for example, a process featuring addition of a wetting agent to microballoons (JP 4-71664 A) and a process characterized by causing an inorganic filler to deposit on surface skins or shells of microballoons. The former process is to impart non-flying property to microballoons by a wetting agent such as a plasticizer, while the latter process is to make microballoons non-flying by fixing an inorganic filler or the like on surface skins or shells of expandable microballoons (which are unexpanded and are filled with an expanding agent) and then causing the unexpanded microballoons. The microballoons with the inorganic filler fixed on the surface skins or shells involve drawbacks such that they are lowered in collapse strength and the non-flying property is imparted at the cost of their performance as a material intended to achieve a weight reduction.
A further approach has also been proposed including, for example, the processes disclosed in JP 4-178442 A and JP 7-196813 A. Each of these processes features expansion of expandable microballoons in a wetting agent. According to the former process, a slurry with expandable microballoons dispersed in a plasticizer is heated to have the expandable microballoons expanded, and subsequent to cooling the expanded microballoons with a fresh supply of the plasticizer as needed, an excess portion of the plasticizer is removed to obtain wet microballoons. According to the latter process, on the other hand, a slurryxe2x80x94which is composed of expandable microballoons and a plasticizer and has been heated to a temperature close to a temperature at which the expandable microballoons start expansionxe2x80x94and a wetting agent (free of expandable microballoons)xe2x80x94which has been heated to the expansion starting temperature of the expandable microballoonsxe2x80x94are mixed to have the expandable microballoons instantaneously expanded, and the thus-expanded microballoons are cooled to obtain wet microballoons.
These conventional processes will be described in further detail. The process disclosed in JP 4-178442 A comprises dispersing, in the form of a slurry, expandable microballoons in a wetting agent, heating the slurry until expansion to have the expandable microballoons expanded, adding a plasticizer for cooling purpose, and finally removing an excess portion of the plasticizer to obtain wet microballoons.
A problem associated with the above-described process is that, because the thermal conductivity of those expanded earlier among expandable microballoons upon expansion under heat becomes extremely low, the heating of the expandable microballoons is prevented to result in a state in which expandable microballoons in an unexpanded form and over-expanded microballoons exist in a mixed state, thereby making it difficult to obtain uniform microballoons. There is another problems in that in the cooling step, the efficiency of heat removal is too low to achieve uniform cooling.
Non-flying microballoons are obtained through the removing step of a wetting agent. As the microballoons have a large surface area and the wetting agent is viscous, it is very difficult to efficiently remove the wetting agent to a target level. Even after the removal of the wetting agent, an excess wetting agent, therefore, remains on the surfaces of the microballoons. This leads to a significant handling problem such that the microballoons have pronounced stickiness and tend to form agglomerates.
On the other hand, the process disclosed in JP 7-196813 A comprises preheating a slurry, in which expandable microballoons are dispersed in a wetting agent, until immediately before expansion; bringing the thus-preheated slurry and a wetting agent, which as a heat source needed for expansion, has been heated to a temperature higher than an expansion temperature, into contact with each other to instantaneously complete the expansion; and then immediately cooling the thus-expanded microballoons in air to obtain microballoons in a form prevented from flying.
This process does not include any factor or cause for the impairment of heat conduction, which is one of the drawbacks of the former process, and therefore, can avoid localized over-heating. Moreover, owing to the use of a wetting agent in a small proportion, cooling is facilitated. It is, therefore, possible to avoid abnormal expansion of expandable balloons and collapse of microballoons, which would otherwise occur due to bursting. Further, occurrence of fusion agglomeration of microballoons themselves can be lessened. However, it is the problems of this process that, because the resin shells forming the expandable microballoons may undergo swelling during the preheating in the wetting agent, a limitation is imposed on the combination of the wetting agent and the resin making up the shells of the expandable microballoons and the preheating has to be conducted at a temperature lower than the expansion starting temperature to avoid the swelling of the resin shells.
As the preheating temperature is lowered, it is necessary either to raise the temperature of the wetting agent as the heat source or to increase the amount of the wetting agent to be used. Because of a deterioration of the wetting agent by heat or safety consideration in the former process and because of the need for the use of the wetting agent in a large amount in the latter process, the wetting agent is obviously used in an amount greater than that needed for effectively preventing flying of microballoons. As a consequence, the microballoons have strong stickiness and tend to form agglomerates, thereby developing inconvenience or a problem in handling.
To form commercially-available, expandable microballoons into a high-flowability slurry in a wetting agent, the wetting agent is usually employed in a proportion 1.5 to 3 times as much as the weight of the expandable microballoons. To this dispersed slurry, a heating, wetting agent as a heat source for having the expandable microballoons expanded is added in substantially the same weight to have the expandable microballoons expanded, so that non-flying microballoons are obtained. As the wetting agent as the expanding heat source has to concurrently heat the wetting agent which forms the dispersed slurry, the wetting agent as the expanding heat source must be added in an amount sufficient to provide heat in a quantity many times over that basically required only for the expansion of the expandable microballoons. Therefore, the wetting agent as the heat source is used in a weight proportion at least 3 times, generally about 5 times as much as the expandable microballoons.
This means that the wetting agent is used in an amount greater than that needed for the prevention of flying of microballoons. The resulting microballoons, therefore, have strong stickiness, thereby developing inconvenience or a problem in handling. Further, the excess use of the wetting agent also develops inconvenience in the cooling step. Described specifically, the resin which makes up the shells of the microballoons is in a softened state after the expansion. The shells themselves, therefore, undergo fusion agglomeration so that large agglomerates are formed. When added to a paint or the like, these agglomerates develop inconvenience such that they rise to the surface. Upon using the paint, it is thus necessary to remove such agglomerates beforehand. Even if removal of such agglomerates is attempted, the screening efficiency is poor due to the stickiness, leading to a problem in that such removal can hardly be achieved industrially.
An object of the present invention is to provide a process for the production of non-flying microballoons, which are free of problems in production such as those described above, difficulties in automated metering and packing work caused by the excessive stickiness of microballoons prevented from flying with a wetting agent, and other problems such as problems in handling upon use, for example, in charging work of expanded microballoons, deposition on the apparatus used, troubles in a metering device, and fouling or contamination of the used apparatus and the production environment.
To achieve the above-described object, the present inventors have proceeded with extensive research. As a result, it has been found that the object of the present invention can be achieved by preheating only expandable microballoons and using, as an expanding heat source for the expandable microballoons, a heated mixture of a wetting agent and gas, leading to the completion of the present invention.
The above-described object of the present invention can be achieved by the present invention as will be described hereinafter. Described specifically, the present invention provides a process for the production of non-flying plastic microballoons, which comprises bringing unexpanded, expandable plastic microballoons, which have been heated to a temperature lower than an expansion starting temperature thereof, and a mixture of a wetting agent and gas, said mixture having been heated to a temperature at least equal to the expansion starting temperature of the expandable plastic microballoons, into contact with each other to cause expansion of the expandable plastic balloons, and then cooling the resulting expanded plastic balloons.
It is the function of the wetting agent to make the microballoons loosely agglomerate by means of the stickiness of the wetting agent such that the microballoons are rendered non-flying. The amount of the wetting agent required to impart non-flying property does not substantially differ in terms of weight ratio to microballoons irrespective of the expansion ratio of the microballoons, because microballoons of a high expansion ratio have a large surface area and are light in weight while microballoons of a low expansion ratio have a small surface area and are heavy in weight. The kind of the wetting agent, in other words, the degree of its stickiness, therefore, has a great effect on the amount of the wetting agent to be used. To 100 parts by weight of microballoons having, for example, a true specific gravity of 0.02 and a mean particle size of 130 micrometers or so, a wetting agent, for example, DINP (plasticizer: diisononyl phthalate) can impart sufficient non-flying property to the microballoons when employed in a proportion of from about 20 to 100 parts by weight. In general, the upper limit of the wetting agent is desirably 300 parts by weight or less.
The process of the present invention can, therefore, impart non-flying property to microballoons by using a wetting agent in a amount substantially smaller than the conventional processes, and can solve the problems of the conventional processes in handling. When microballoons produced by the conventional processes are added to paints, inks, sealants and the like, the microballoons give influence to the physical properties of the paints, inks, sealants and the like due to the inclusion of a wetting agent in a large proportion. The process of the present invention also makes it possible to reduce such influence.
The present invention can reduce the amount of a wetting agent, and can provide non-flying microballoons improved also in handling properties. The microballoons obtained by the process of the present invention has a merit in that no limitation is imposed on their application owing to the inclusion of the wetting agent in a smaller proportion. Described specifically, the wetting agent adhered on the non-flying microballoons, in many instances, is not always identical in composition to a plasticizer or the like in a material in which the microballoons are used as a weight-reducing material. In the case of conventional non-flying microballoons which contains a wetting agent in a large proportion, a limitation is imposed on their application, or it becomes necessary to design non-flying microballoons with the kind of a wetting agent varied depending upon their application. These limitations are troublesome both technically and industrially in many instances, and the production process of the present invention, which does not develop such problems, has a significant industrial merit in that the amount of a wetting agent can be reduced.